Deposition of sediment inside the conduit structures; eg inverted siphon; reduces the flow cross section area and increase the flow roughness which causes reduction of flow discharge passing through the conduit. In order the structure to function properly, the structure should design not to allow any sediment particle to deposit. Or if deposited, it can be possible to remove them by the action of flow. Among the methods in which can be achieved such goals is to design the structure in such a way that the flow velocity to be greater than incipient motion velocity. There are many relations to obtain the incipient motion velocity of sediment particles on the bed or slope of an open channel. However for the case of closed conduits under low pressure little information is available. Such criterion is useful for sewer cleaning too. To determine the criteria for determining the incipient motion velocity at closed box conduits this study was conducted. To do so first a general relationship was developed by applying the dimensional analysis and then experimental tests were conducted to obtain data. In these tests five different sediment particle sizes, 0.5, 0.7, 1.0, 2.0 and 3.2 millimeters under four different bed slopes 0.0, 8.0, 16, and 24 degrees were conducted. By analyzing of these data design charts were developed. From these curves, the incipient motion velocity can be determined. The results were compared with the results of other investigators. The application of this study in some of the inverted siphons in Khuzestan province of Iran has been presented.
)
